feat(cli): add 'terminal-agent export' command for Markdown and HTML Proof-of-Done certificates

[Kaap10]

Problem Statement

The Proof of Done audit details, test metrics, and failure recovery traces are currently stored as raw JSON inside .terminal_agent/sessions/<id>.json. There is no command to export these results into a formatted GitHub pull request description or HTML report for CI pipelines and code reviews.

Proposed Architecture & Solution

1. Export Command (src/terminal_agent/cli/commands/export.py):
   • Command syntax:

     terminal-agent export [SESSION_ID] [--format markdown|html|json] [--output path]

   • If SESSION_ID is omitted, automatically defaults to the latest session.
2. Report Contents:
   • Task Contract Summary: Goal, constraints, and success criteria.
   • Verification Results Table: Tests executed, tests passed/failed, assertion checklist.
   • Git Diff & Files Changed: Summary statistics and file modification breakdown.
   • Autonomous Recovery Timeline: Failures classified and repair hypotheses applied.
   • Tamper-Evident Proof of Done Certificate: Session ID, timestamp, and verification badge.
3. HTML / Markdown Templates:
   • Markdown format optimized for direct copy-pasting into GitHub PR descriptions and issue summaries.
   • HTML format styled with a clean dark/light theme suitable for CI artifacts.

Files to Modify / Create

• src/terminal_agent/cli/commands/export.py (New)
• src/terminal_agent/cli/main.py (Register export command)
• src/terminal_agent/session/manager.py (Add report synthesis helper)
• tests/unit/test_export_command.py (New)

Acceptance Criteria

• terminal-agent export generates valid GitHub-flavored Markdown by default.
• terminal-agent export --format html --output report.html writes a standalone HTML report.
• Missing or invalid session IDs output an informative error message and exit cleanly.
• Unit tests verify Markdown and HTML output structure and contents.